The Third Generation Partnership Project (3GPP) has developed standards for a cellular communications system, referred to as Long Term Evolution (LTE). LTE is based on transmission using Orthogonal Frequency Division Multiplexed (OFDM) signals.
An OFDM signal comprises a plurality of subcarriers, each of which is modulated to carry a transmission symbol. Data for transmission is divided into a plurality of parallel streams, one for each subcarrier. The transmission symbol for each subcarrier is generated by modulating the data using a conventional modulation scheme. Advantageously, OFDM simplifies channel equalisation, by adapting the modulation scheme applied to each sub carrier according to the narrowband propagation conditions for that subcarrier.
In cellular systems, such as LTE, a mobile station estimates the downlink radio channel in order to coherently demodulate information transmitted to it from the base station. In LTE, the base station inserts reference symbols (RS) into the OFDM signal that it transmits. These are used for channel estimation by the mobile stations. At least one cell-specific reference symbol is transmitted by a base station in a given time period.
3GPP standard TS 36.211 Release 9 defines the transmission of reference symbols in the downlink. The reference symbols are transmitted over the full downlink channel bandwidth for the cell covered by the base station. Mobile stations measure the reference symbols transmitted by the base station providing them with service. They additionally measure the reference symbols transmitted by other base stations. This is useful for handover purposes, when moving between cells covered by different base stations.
A mobile station can also use the reference symbols to estimate the channel for resource blocks (that is, transmission symbols allocated to a specific subcarrier) that are not being used to receive data. This information is reported to the base station and typically used by it to schedule downlink resources optimally to mobile stations.
When there is insufficient data to fill the capacity of the OFDM downlink signal, it is desirable to improve the efficiency of the transmission without reducing the link quality for any subscriber stations.